1. Field of the Invention
The present invention relates to optical waveguide gratings and production methods therefor.
2. Description of the Related Art
Optical waveguide gratings can be obtained by making regular periodic changes, such as periodic changes in the refractive index of the core, along the longitudinal direction of optical fibers or planar optical waveguides.
In general, gratings can be divided into radiative mode-coupled types and reflective mode-coupled types. Radiative mode-coupled gratings are capable of attenuating light of specific wavelengths due to radiation from the optical waveguide by coupling modes propagating in the core with modes propagating in the cladding. Reflective mode-coupled gratings reflect light of specific wavelengths by coupling modes propagating through the core in a positive direction and modes propagating through the core in the opposite direction (negative direction).
For example, in the case of optical fiber gratings formed by making periodic changes in the core refractive index of optical fibers, radiative gratings are obtained by making the period of the changes in the core refractive index (hereinafter sometimes referred to as the grating pitch) approximately several hundred microns, and reflective gratings are obtained by making the grating pitch approximately 1 micron.
Radiative mode-coupled gratings have wavelength-transmission loss properties (transmission spectra) as shown in FIG. 4, wherein the transmission loss of light in a specific wavelength band is selectively increased. The width of the wavelength band with an increased transmission loss is referred to as the rejection bandwidth, the central wavelength thereof is referred to as the central wavelength of the rejection band, and the magnitude of the change in transmission loss is referred to as the rejection.
Wavelength-transmission loss properties (transmission spectra) similar to those of FIG. 4 can also be obtained by reflective mode-coupled gratings.
These grating properties of optical waveguide gratings are known to change with the parameters of the gratings, i.e. the amount of change in the core refractive index, the grating pitch, the grating shape (profile of the core refractive index), the grating length in the longitudinal direction of the optical fiber, and the effective refractive index.
The following Table 1 summarizes the influence that each parameter of a grating has on the grating properties. In the table, .times. indicates no influence, .smallcircle. indicates some influence, and .DELTA. indicates a small influence. Additionally, the arrows .uparw. (.dwnarw.) indicate whether the value of the cladding property will increase (decrease) in response to an increase in the parameter value.
Precise control of the central wavelength and the rejection are important when using the optical waveguide gratings as optical components in optical communication systems.
TABLE 1 ______________________________________ Central Rejection PARAMETER Wavelength Rejection Bandwidth ______________________________________ Change in Refractive Index .smallcircle..uparw. .smallcircle..uparw. x Grating Pitch .smallcircle..uparw. .DELTA. x Grating Shape .smallcircle. .smallcircle. x Grating Length x .smallcircle..uparw. .smallcircle..dwnarw. Effective Refractive Index .smallcircle..uparw. x x ______________________________________
As conventional methods for making periodic changes in the core refractive index of optical waveguide gratings, there are methods which take advantage of the properties of silica glass doped with germanium, of which the refractive index will increase when exposed to strong UV radiation, depending on the amount of exposure.
For example, methods are known wherein a silica glass based optical fiber having a core doped with germanium oxide is hydrogenated in a hydrogen-pressurized container (at 100 atm), and then either exposed to UV radiation at a constant period along the longitudinal direction of the optical fiber using a photomask, or exposed to UV radiation at regularly spaced intervals along the longitudinal direction of the optical fiber.
FIG. 7 shows an example of a conventional optical fiber grating production apparatus.
In the drawing, reference numeral 11 denotes an excimer laser, which can generate UV light having a wavelength of 248 nm. Reference numeral 12 denotes an optical system, which is constructed to increase the spot size of the UV light emitted from the laser 11 to a predetermined size on the order of millimeters to tens of millimeters. Reference numeral 13 denotes a metallic mask, having slits which are cut at regularly spaced intervals on the order of tens to hundreds of microns. Reference numeral 14 denotes an anchoring block to which is anchored an optical fiber 1 having the jacket layer 4 partially removed. The UV light emitted from the laser 11 is directed by the optical system 12 through the mask 13 so as to irradiate the portion of the optical fiber 1 with the jacket layer 4 removed.
In order to make an optical fiber grating using an apparatus having this type of structure, the UV light should be emitted from the laser 11 so as to irradiate the optical fiber 1 through the mask 13. As a result, the core refractive index is raised at only the portions of the optical fiber 1 which have been exposed to UV light, so as to form a grating portion 5 in which the core refractive index periodically changes.
However, while this conventional production method requires the grating properties to be controlled by means of the shape of the mask 13 and the UV light irradiation conditions, such control is complicated because the central wavelength and the rejection cannot be independently controlled.
For example, with the above-mentioned production method, it is necessary to find the optimum combination for the shape of the mask 13 (grating pitch, grating shape) and the UV light irradiation conditions (UV light intensity, irradiation time) before forming the grating portion. In order to determine the shape of the mask 13 to use, a number of different irradiation conditions are tried on a first mask 13, and if the desired central wavelength and rejection are not able to be obtained, then the shape of the mask 13 is changed and the irradiation conditions are tried on the new mask. This process of trial and error must be repeated many times, so that a lot of time is expended in order to select the shaped of the mask 13 and the irradiation conditions.
Particularly in the case of radiative mode-coupled gratings, several masks must be used in order to adjust the central wavelength and rejection to their desired values because the amount of change in the central wavelength is large in comparison to the amount of change in the core refractive index.
Additionally, the monitoring of the transmission properties during the UV light irradiation process is difficult because the amount of change in the refractive index changes over time, i.e. the transmission properties (central wavelength and rejection) change over time, until the hydrogen is removed by dehydrogenation.
Furthermore, it is impossible to irradiate the mask with a uniform UV beam because the spatial distribution of the intensity of a laser beam is not uniform, i.e. the laser beam intensity within the spot of a laser is non-uniform. For this reason, the core refractive index change in each part of the grating portion 5 cannot be made uniform, so as to make the transmission spectra of the optical fiber gratings broader or asymmetric, thereby degrading the grating properties.
FIG. 8 shows another example of a conventional optical fiber grating production apparatus. The elements which are identical to those in FIG. 7 are given the same reference numerals and their explanations will be omitted below.
Reference numeral 22 denotes an optical system which is constructed so as to converge the UV light emitted from the laser 11 to a predetermined spot size on the order of tens to hundreds of microns. Reference numeral 24 denotes a mobile stage to which is affixed an optical fiber 1 having the jacket layer 4 partially removed. The mobile stage 24 is controlled so as to be capable of jogging by predetermined distances along the longitudinal direction of the optical fiber. The UV beam emitted from the laser 11 passes through the optical system 12 to irradiate the portion of the optical fiber 1 having the jacket layer 4 removed.
In order to produce an optical fiber grating using an apparatus of this structure, a UV beam is first emitted from the laser 11 for irradiation of the optical fiber 1. After irradiation for a standard period of time, the mobile stage 24 is jogged in order to shift the optical fiber 1 in the longitudinal direction by a predetermined distance, then the optical fiber 1 is once again exposed to the UV beam for a standard period of time. This process of irradiation with the UV beam followed by shifting of the optical fiber 1 is repeated until the total distance by which the optical fiber 1 has shifted has reached a predetermined distance, thereby completing the process. In this way, the grating portion 5 can be formed because the core refractive index is changed at only the portions of the optical fiber 4 which are exposed to the UV beam.
According to this method, the core refractive index change in the grating portion 5 can be made uniform by repeatedly duplicating UV beam irradiation under identical irradiation conditions. However, since the grating pitch is determined by the spot size of the laser beam and the shifting pitch of the optical fiber 1, the spot size of the laser beam and the shifting pitch of the optical fiber 1 must be changed in order to change the grating pitch. While the spot size of the laser beam can be changed by using slits, lenses, or a combination of these, it is difficult to precisely control the spot size by means of any of these methods. Therefore, there is a drawback in that a complicated process of finding the laser beam irradiation conditions is required in order to change the grating properties, thus making design changes difficult.
On the other hand, reflective mode-coupled gratings can achieve wavelength-reflected optical intensity loss properties (reflection spectra) such as shown in FIG. 20, wherein the reflected optical intensity of light in a specific wavelength band is selectively increased. The width of this wavelength band with an increased reflected optical intensity is referred to as the reflection bandwidth, of which the central wavelength is referred to as the central wavelength of the reflection band, and the proportion of the reflected optical intensity with respect to the incident optical intensity is referred to as the reflection.
These grating properties are known to change according to the parameters of the grating such as the amount of change in the core refractive index, the grating pitch, the grating shape (profile of core refractive index change) the grating length along the longitudinal direction of the optical fiber, and the effective refractive index. The effective refractive index is a refractive index which is locally averaged along the longitudinal direction and the transversal direction of the core of the optical fiber, which can be experimentally determined by the following relation: 2.pi./.LAMBDA..multidot.effective refractive index=propagation constant at central wavelength of reflection band (wherein .LAMBDA. is the grating pitch).
In this case, "locally along the longitudinal direction of the optical fiber" signifies a region having a length of about a single period of the grating.
FIG. 21 shows a closeup of the relevant portions when using a phase mask with the method shown in FIG. 7.
For example, one known method for making a reflective mode-coupled grating is to hydrogenate a silica optical fiber 31 having a core 31a doped with germanium oxide in a hydrogen-pressurized container (at 100 atm), then to expose the optical fiber 31 to a UV beam 34 via a phase mask 32 composed of a silica glass plate having a plurality of slits formed therein at regular intervals. According to this method, when the UV beam 34 is orthogonally incident on the top surface of the phase mask 32, the diffracted light forms an interference pattern. This interference pattern causes periodic changes in the intensity of the UV beam 34 irradiating the optical fiber 31, so as to form a grating portion 310 having periodic changes in the refractive index of the core 31a.
As another technique for making reflective mode-coupled gratings, there is a technique referred to as apodization, wherein the core refractive index change is gradually varied along the longitudinal direction of the optical fiber 31. For example, if the change in the core refractive index at the grating portion 310 due to exposure to the UV beam 34 is constant along the longitudinal direction of the optical fiber 31, then anomalous ripples will occur around the central wavelength in the reflection spectrum as shown in FIG. 22. Apodization is a process for preventing the occurrence of these anomalous ripples by varying the core refractive index change in the grating portion 310 along the longitudinal direction of the optical fiber 31 so as to outline the curve shown in FIG. 23, instead of making the core refractive index change constant along the longitudinal direction of the optical fiber 31. In the graph of FIG. 23, the solid lines indicate the core refractive index change along the longitudinal direction of the optical fiber 31 and the dashed curve indicates the distribution of the core refractive change along the longitudinal direction of the optical fiber 31.
The shape of the distribution of the core refractive index change can be made into a variety of shapes without being restricted to the example of FIG. 23, and the reflection properties of the reflective mode-coupled gratings are known to change depending on the differences in the shape of the distribution of the core refractive index change (see BSTJ., vol. 55, pp. 109-126, 1976, "Filter Response of Nonuniform Almost-periodic Structures", H. Kogelnik et al.).
Specifically, as methods for performing apodization during the formation of the grating portions such as the method wherein the grating portions are formed by using a phase mask 32 as explained above, there is a method wherein the UV beam is shifted along the optical fiber 31 while changing the irradiation intensity of the UV beam 34, or a method wherein the optical fiber 31 and phase mask 32 are shifted in the longitudinal direction of the optical fiber 31 while changing the irradiation intensity of the UV beam 34 and holding the UV beam 34 stationary.
However, in the case wherein the UV beam 34 is moved, the structure of the optical system in the production apparatus becomes complicated, so that the costs required for production such as those for temperature and humidity regulation rise and it becomes extremely difficult to shift the UV beam 34 with precision.
On the other hand, in the case wherein the optical fiber 31 and the phase mask 32 are moved, the optical fiber 31 and phase mask 32 are affected by the vibrations of drive devices such as motors. Particularly when forming the grating portion using a phase mask 32, the positional precision of the phase mask 32 and the optical fiber 31 must be in submicron units. For this reason, when the optical fiber 31 and the phase mask 32 are moved, extreme care is necessary in order to stabilize them to prevent mutual displacement of their relative positions, thus requiring the use of expensive fixation devices and increasing the number of production steps.